Stimulation method

ABSTRACT

The present invention relates to a stimulation system for stimulation of oil production in an oil field. The stimulation system comprises a plurality wells, wherein a plurality of activation devices are arranged in the wells, and wherein the activation devices are activated with a frequency of once within a period of 1-365 days and with an energy discharge of at least 0.1 kilograms TNT equivalence per activation. Furthermore, the invention relates to a stimulation method.

FIELD OF THE INVENTION

The present invention relates to a stimulation system for stimulation of oil production in an oil field. Furthermore, the invention relates to a stimulation method.

BACKGROUND ART

In the recovery of hydrocarbon-containing fluid, such as oil, from hydrocarbon-bearing reservoirs, it is usually possible to recover only a limited part of the oil in the reservoir by so-called primary recovery methods which utilise only the natural forces present in the reservoir. A variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is water-flooding which involves the injection of water into the reservoir from an injection well. As the water moves through the reservoir, it acts to displace or flush the oil therein towards a production well through which the oil is recovered. During recovery of hydrocarbon-containing fluid, reservoir pressure is thus maintained by injecting water from injection wells 1 surrounding the production wells 2. The water cut of the recovered hydrocarbon-containing fluid is measured on a regular basis in production wells 2 to detect water breakthrough. The water may come from the injection well or may be water which is naturally occurring from the reservoir. In order to avoid water breakthrough and enhance production, it has been attempted to use so-called secondary recovery methods using other drive fluids, such as CO2, methane gas or similar fluids that are often miscible in hydrocarbons.

Another way of enhancing production of hydrocarbons in the recovered fluid is to use stimulation of the reservoir. The stimulation process comprises the use of tools and is rarely initiated before it is absolutely necessary, e.g. when the water cut is above a certain level, e.g. 90% water. Known stimulations tools send out mechanical vibrations into the reservoir when the water cut is increasing or is above a predetermined level. The tool for emitting the vibrations is then submerged into the production well to the point approximately opposite the production zone while the production is set on hold. The production is then resumed when stimulation has been completed. Stimulation tools may also be arranged in the injection well so that production can continue during the stimulation process. Enhancement of hydrocarbon recovery by mechanical stimulation is difficult, time consuming and extremely expensive, especially since deep wells become increasingly widespread in the extraction of oil.

SUMMARY OF THE INVENTION

It is an object of the present invention to wholly or partly overcome the above disadvantages and drawbacks of the prior art. More specifically, it is an object to provide an improved stimulation method optimising the stimulation of the reservoir.

The above objects, together with numerous other objects, advantages, and features, which will become evident from the below description, are accomplished by a solution in accordance with the present invention by a stimulation system for stimulation of oil production in an oil field, comprising

-   -   a plurality of wells, and     -   a plurality of activation devices arranged in the wells,         wherein the activation devices are activated with a frequency of         once within a period of 1-365 days and with an energy discharge         of at least 0.1 kilograms TNT (trinitrotoluene) equivalence per         activation.

In an embodiment, the activation devices may be reusable, i.e. the activation devices can be used several times, eliminating the need to pull the activation devices out of the well for recharging.

The activation devices may be activated with the frequency of once within a period of 1-185 days, preferably once within a period of 1-90 days, more preferably once within a period of 1-30 days, and even more preferably once within a period of 5-20 days.

Also, the activation devices may be activated repeatedly, once in each period, wherein the period is repeated several times.

Moreover, the activation devices may be activated at intervals of at least 5 days, preferably at intervals of at least 10 days, more preferably at intervals of at least 15 days.

The activation devices being activated “with a frequency of once within a period” shall be construed to mean that each activation device of the plurality of activation devices is activated once within a period as specified, and “the period being repeated several times” shall be construed to mean that said period as specified subsequently may be repeated several times. This means that according to the present invention, each activation device of the plurality of activation devices is activated once within a period as specified, and once said period is over, the activation device is activated again, and the period as specified is thus repeated several times. By a frequency is meant that the activation devices are activated at least twice, each activation being performed within the period and the period thus being repeated several times. Hence, the activation devices are activated once in each period and not twice a day or several times within the same period.

In the stimulation system as described above, the wells may be both a plurality of production wells and a plurality of injection wells, the plurality of activation devices being arranged in the injection wells and/or production wells.

Said activation devices may be activated with the frequency of once within the period of 1-185 days, preferably within the period of 1-90 days, more preferably within the period of 1-30 days, and even more preferably within the period of 5-20 days.

Also, the activation devices may be activated with the energy discharge of at least 0.5 kilograms TNT equivalence per activation, preferably at least 1 kilograms TNT equivalence per activation, more preferably at least 5 kilograms TNT equivalence per activation.

In an embodiment, a first activation device of the plurality of activation devices may be activated before a second activation device of the plurality of activation devices.

Said first activation device may be determined as the activation device nearest to the production well in which water cut is increasing.

Moreover, the first and second activation devices may be activated on the same day, or even simultaneously.

Further, the first activation device may be activated on a first day of the period, and the second activation device may be activated on another day of the period.

Also, the activation device may be a fluid-activated gun, said fluid being pressurised injection fluid, and the gun may convert energy from the pressurised fluid into mechanical waves, where said gun is activated continuously in a time interval during the period, providing vibrations having an energy of at least 0.1 kilograms TNT equivalence in total during the period.

The gun may emit electromagnetic pulses of electromagnetic radiation.

The gun may also be an electromagnetic hammer.

In addition, the gun may be activated continuously during the period.

Furthermore, at least part of the plurality of activation devices may be arranged in the plurality of injection wells, said injection wells encircling at least one production well.

In another embodiment, said injection wells may encircle a plurality of production wells.

Additionally, at least part of the plurality of said activation devices may be arranged in a plurality of periphery injection wells, said periphery injection wells encircling at least one production well and at least one non-periphery injection well.

Moreover, the activation devices may be activated in a predetermined pattern determining in which injection well and/or production well the activation device is activated.

By pattern is meant the order of the wells in which an activation device is activated.

The activation device may consist of at least one member selected from the group of downhole perforation guns, fluid-activated guns, seismic sources and transducers, chemical reaction guns or solid fuel guns.

Such solid fuel guns may comprise solid fuel, such as charcoal, graphite or cordite, and potassium nitrate or sodium nitrate. The solid fuel may also be mixed with sulphur.

In an embodiment, the perforation gun may comprise non-perforating charges.

Also, the activation devices may be activated simultaneously to the injection of an injection fluid from at least an injection well towards the at least one production well.

Further, the injection fluid may have a temperature at a point of injection downhole which is higher than that of the formation.

The temperature of the hot fluid may be at least 10° C. higher than the temperature of the formation, preferably at least 25° C. higher than the temperature of the formation, and more preferably at least 50° C. higher than the temperature of the formation.

In another embodiment, the temperature of the hot fluid may be at least 150° C., preferably at least 175° C., and more preferably at least 200° C.

Moreover, the injection fluid may be a fluid selected from a group consisting of gas, such as methane gas, carbon dioxide, nitrogen gas and water, or other liquids.

The stimulation system as described above may further comprise a plurality of openings in at least one of the wells, wherein at least two neighbouring openings have different inlet flow settings, wherein the activation device may be arranged between said two neighbouring openings having different inlet flow settings for transmission of mechanical waves into a region of the formation having a high pressure gradient, thereby releasing oil in said region.

Further, inlet valves may be arranged in the openings and at least two neighbouring valves may have different inlet flow settings, wherein the activation device may be arranged between said two neighbouring valves having different inlet flow settings for transmission of mechanical waves into a region of the formation having a high pressure gradient, thereby releasing oil in said region.

The present invention further relates to a stimulation method comprising the steps of:

-   -   arranging a plurality of activation devices in a plurality of         wells, and     -   activating the activation devices with a preselected range of         frequencies or a single frequency.

Additionally, the activation devices may be arranged in injection wells and/or production wells, the injection wells and/or production wells encircling at least one production well.

In an embodiment, a first activation device arranged in a first well may be activated in the first well with a frequency of once within a period of 1-365 days, and a second activation device arranged in a second well may be activated in the second well with a frequency of once within a period of 1-365 days.

Also, the activation devices may be activated with an energy discharge of at least 0.1 kilograms TNT equivalence per activation.

Further, the activation devices may be activated with a frequency of once within a period of 1-365 days.

Moreover, the activation devices may be activated with a frequency of once within a period of 1-185 days, preferably once within a period of 5-90 days, more preferably once within a period of 7-30 days, and even more preferably once within a period of 7-20 days.

The stimulation method as described above may also comprise the step of activating the activation devices in a predetermined pattern determining in which well an activation device is activated.

Also, the stimulation method as described above may comprise the steps of:

-   -   activating a first activation device of the plurality of         activation devices encircling at least one production well,     -   activating a second activation device positioned substantially         furthest away from the first activation device and on the         opposite side of the at least one production well,     -   activating a third activation device positioned substantially         furthest away from the second activation device and on the         opposite side of the at least one production well, and     -   activating a fourth activation device positioned substantially         furthest away from the third activation device and on the         opposite side of the at least one production well and so forth         until all activation devices of the plurality of activation         devices are activated, and then activating the plurality of         activation devices once more a predetermined number of times.

Further, the stimulation method as described above may comprise the step of activating all activation devices of the plurality of activation devices encircling at least one production well and then activating any of the activation devices once more.

In addition, the first activation device may be activated on a first day of the period, and the second activation device may be activated on another day of the period.

The activation devices may be a fluid-activated gun, said fluid being pressurised injection fluid, and the gun may convert energy from the pressurised fluid into mechanical waves, where said gun is activated several times during the period, providing vibrations having an energy of at least 0.1 kilograms TNT equivalence in total during the period.

Moreover, the activation devices may consist of at least one member selected from the group of downhole perforation guns, fluid-activated guns, seismic sources and transducers, chemical reaction guns or solid fuel guns.

Also, the injection fluid may have a temperature at a point of injection downhole which is higher than that of the formation.

The stimulation method as described above may further comprise the step of arranging a plurality of activation devices in a plurality of periphery injection wells, said periphery injection wells encircling at least one production well and at least one non-periphery injection well.

In addition, the stimulation method as described above may further comprise the steps of:

-   -   determining a water cut in a production well, and     -   increasing the activation frequency of the activation devices if         the water cut is above a preselected level, or     -   decreasing the activation frequency of the activation devices if         the water cut is below a preselected level.

Finally, the stimulation method as described above may further comprise the steps of:

-   -   setting an inlet flow of a plurality of inlet valves in openings         in a first production zone so that inlet valves in openings in a         second and neighbouring production zone have different inflows,         thereby creating a pressure gradient in a region of the         formation between said plurality of inlet valves, and     -   arranging and activating the activation devices in the well         opposite the region of the formation between said plurality of         inlet valves having different inflows, thereby releasing oil in         said part of the formation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its many advantages will be described in more detail below with reference to the accompanying schematic drawings, which for the purpose of illustration show some non-limiting embodiments and in which

FIG. 1 shows an oil field seen from above,

FIG. 2 shows a stimulation system seen in perspective illustration,

FIG. 3 a shows an injection well and a production well before activation of an activation device,

FIG. 3 b shows the wells of FIG. 3 a after activation of the activation device,

FIG. 4 a shows an injection well and a production well before activation of an activation device,

FIG. 4 b shows the wells of FIG. 4 a after activation of the activation device,

FIG. 5 shows an activation device in an injection well discharging energy towards a production well,

FIG. 6 shows another oil field seen from above,

FIG. 7 a shows the arrangement of the activation device between two production zones in a production well, and

FIG. 7 b shows the arrangement of the activation device between injection zones in an injection well.

All the figures are highly schematic and not necessarily to scale, and they show only those parts which are necessary in order to elucidate the invention, other parts being omitted or merely suggested.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an illustration of an oil field 101 seen from above, comprising two production wells 2, 2 a, 2 b and six injection wells 1, 1 a, 1 b, 1 c, 1 d, 1 e, 1 f. The invention relates to a stimulation system 100 comprising a plurality of wells and a plurality of activation devices 3 (shown in FIG. 2) arranged in the wells. The activation devices are arranged each in a well in the oil field 101, where each well may be an injection well 1 and/or a production well 2. Thus, the activation devices may all be arranged in production wells or may all be arranged in injection wells, or a combination thereof. In order to stimulate the oil production on a regular basis, the activation devices are activated with a frequency between 1 and 365 days and with an energy discharge of at least 0.1 kilograms TNT equivalence per activation. Thus, the activation period is 1-365 days before the activation device is activated again.

FIG. 2 shows a stimulation system 100 for stimulation of oil production in the oil field 101. The stimulation system 100 comprises a plurality of injection wells 1 a, 1 b, 1 c, 1 d, 1 e, 1 f, a plurality of production wells 2, 2 a, 2 b having production zones 10 a, 10 b with openings and a plurality of activation devices 3, 3 a, 3 b, 3 c, 3 d, 3 e, 3 f arranged in the injection wells 1.

By stimulating the oil field 101 at a predetermined frequency, the production is stimulated on a regular basis and not just when the water cut is increasing. The pools of oil, i.e. subsurface oil accumulations in the rock, such as limestone, sandstone or shale, filled with small oil-filled micro bores, are then affected continuously by the discharged energies, and the production of oil from the formation is enhanced. Simultaneously, the low frequency mechanical stimulation initiates micro-fracturing of the formation, especially in limestone formation but also in sandstone and other types of oil-bearing formations, or even collapses of micro-cavities in the formation containing oil, gas or a mixed fluid, thereby changing the pressure regime in the formation and displacing the fluids towards the production wells. The micro bores created by the stimulation enable the oil to flow and accumulate in larger pools or areas of oil-containing fluid. By injecting an injection fluid simultaneously to the stimulation of the reservoir by mechanical stimulation, the larger pools or areas of oil-containing fluid may be forced towards the production wells 2 close to the injection wells. Stimulation and injection are not necessarily performed simultaneously, but interchanging patterns of stimulation and injection may also be equally effective, since the velocity of propagation is very different between injection, e.g. water penetration propagation, and stimulation, e.g. mechanical wave propagation.

The activation frequency of the stimulation system of FIG. 2 may be that one activation device is activated in one well every 6 days, where a first activation device 3 a of a first injection well 1 a is activated on day one. On the second day, the activation device opposite the production wells 2 a, 2 b and most remote from the first activation device 3 a is activated. The activation device 3 not already activated, opposite the production wells 2 a, 2 b and most remote from the second activation device 3 b of the second injection well 1 b, is activated on the third day. On the fourth day, the fourth activation device 3 d of the fourth injection well 1 d is activated since this activation device is the activation device furthest away from the third activation device 3 b and opposite the production wells 2 a, 2 b, which is not activated in this activation period. Then the fifth activation device 3 e of the fifth injection well 1 e is activated, and finally the sixth activation device of the sixth injection well is activated. Thus, the activation period is 6 days during which all activation devices involved are activated once. The sequence of activations 3 a, 3 f, 3 b, 3 d, 3 c, 3 e resembles an alternating “star” pattern sequence, also known from other technical fields such as from the tightening of bolts on car wheels, flanges etc. and may ensure the most optimal stimulation sequence for entrapping oil between a set of injection wells and forcing the oil towards one or more production wells centered within the set of injection wells.

By having more injection wells, the sequence becomes longer but the pattern is still similar, i.e. the activation device furthest away from the production well is picked, and subsequently the activation device furthest away, opposite the production wells and not yet activated in the current sequence is picked. Furthermore, sequences may be superimposed or suboptimised due to specific knowledge of characteristics of a given formation.

By repeating a predetermined pattern of activation, the production may be stimulated continuously and not just when the water cut has increased to above a certain level. Hereby, the energy resource for recovering the hydrocarbon-containing fluid is utilised in a more optimal manner than when stimulation is only initiated above a predetermined water cut level. In the latter case, energy is then used for recovering an unnecessary amount of water while continuous stimulation keeps the water content and therefore also the energy used for bringing up water at a minimum.

When injecting fluid 21 into the formation to keep reservoir pressure and to drive hydrocarbon-containing fluid, such as oil, towards the production wells 2, the area/volume of oil is changed or displaced. The oil-containing area 20 may be split into several areas as shown in FIG. 3 a, or the area may no longer occur as a level horizontal layer as shown in FIG. 4 a. The oil-containing area 20 may therefore be displaced relative to a production zone 10 in the production well 2, so that the production well produces oil-containing fluid with a water cut which is too high. By discharging energy with a predetermined frequency, e.g. once every 6 days for one activation device in an oil field of six injection wells 1, the area containing oil accumulates again, so the injection fluid 21 pushes the oil-containing area 20 from one side as shown in FIG. 3 b, or levels out so the injection fluid pushes the oil-containing area from below, as shown in FIG. 4 b. The energy discharged from the activation device is thus transmitted to oil-containing parts of the formation which may then accumulate oil in larger areas. When activating the surrounding injection wells 1 of a production well, the oil-containing fluid is accumulated in a large area surrounding the production well and oil-containing fluid can thus flow into the production well again. If only one injection well is activated without activating several of the other surrounding injection wells 1 of the production well, the injection fluid from the other injection wells 1 flows into the production well and takes over, keeping the oil-containing fluid from flowing into the production well. It is therefore important that more than one of the surrounding injection wells 1 of the production well are activated to force the oil-containing fluid towards the production well and to surround the production well, so that the injection fluid leaves the oil-containing fluid to act as drive fluid.

By activating the oil field continuously from various injection wells 1 and/or production wells 2, the oil-containing fluid is helped accumulate in larger areas. Furthermore, the energy discharge provides micro bores in the formation in areas or collapses in micro-cavities, where an adequate pressure gradient is present, and thus helps the oil-containing fluid trapped in pockets to flow and accumulate into larger areas of oil-containing fluid.

The activation device is controlled to discharge energy in a predetermined pattern determining in which injection well and/or production well the activation device is activated. Some of the activation devices may be activated more frequently than others, and two different activation devices may even be activated on the same day. The activation device/devices being activated more frequently than some of the others is/are the first activation device/devices determined as the activation device/devices nearest to the production well in which water cut is increasing.

When the water cut is increasing, the activation devices 3 are activated more frequently in the predetermined pattern, or the pattern is changed. If the water cut still increases, the pattern is changed so that the activation device nearest to the production well, in which the water cut is increasing, is activated more frequently than others, or the pattern is maintained and the frequency is increased until the water cut is decreasing again.

The activation devices may be arranged both in the injection wells 1 and the production wells 2. By arranging the activation devices in the production wells 2, the source of the energy is closer to the area to be activated. However, it may disturb the production of that production well. By arranging the activation devices in the injection wells 1, the source may be further away from the area to be activated. Hence, this activation device does not disturb production, and when using some activation devices, e.g. a fluid-activated gun, the injection of injection fluid or drive fluid is not prevented either.

In FIG. 5, the activation device 3 of the stimulation system is a fluid-activated gun in which the injection well is pressurised with injection fluid in order to activate the gun and inject fluid into the reservoir at the same time and thereby convert energy from the pressurised fluid into mechanical waves. The gun is activated substantially continuously in an interval during the activation period, providing vibrations having a total energy of at least 0.1 kilograms TNT equivalence during the period of 1-365 days. By having a fluid-activated gun in comparison to an explosive-activated gun, the activation device needs to be activated more frequently than the explosive-activated gun due to the fact that in one discharge, the perforating gun discharges much more energy than what is possible for a fluid-activated gun. However, the explosive-activated gun needs to be reloaded on a regular basis, disturbing the production if the gun is arranged in the production well. The fluid-activated gun is arranged in the injection well and does not need to be reloaded, and it does not necessarily disturb the flow in the well. When using explosives, the production well is often closed while performing the activation as a safety precaution, and thus the production is set on hold while performing stimulation.

Thus, the activation device may be a downhole perforation gun, a fluid-activated gun, a seismic source, a chemical reaction gun or a solid fuel gun. The perforation gun may comprise non-perforating charges, and thus be a non-perforating gun. The activation device may also be an electromagnetic hammer.

The fluid activated gun may be a gas-activated gun, and thus the injection fluid 3 is gas, such as methane gas, carbon dioxide or nitrogen gas. In one embodiment, the gas accumulates in a piston chamber in the gun, driving a piston in one direction in the chamber compressing a spring, and when the spring cannot be compressed any further, a release mechanism is activated and the piston moves at a high velocity in the opposite direction, hammering into the back wall of the chamber creating the mechanical waves. In another embodiment, the gas gun is activated by pulsed injection fluid 3, creating the hammering effect for the generation of mechanical waves. The gun may also emit electromagnetic pulses of electromagnetic radiation.

The chemical reaction gun is a gun in which at least two chemicals react to vaporise and thus provide mechanical waves travelling into the formation. The chemicals may be sent down in two flow lines, each supplying a chemical which is mixed in the gun. The chemicals may be the two gases oxygen and methane or potassium permanganate and dichromate. One or all of the chemicals that are to react may also be present in the gun from the beginning, working as an oxidant, such as potassium dichromate or potassium permanganate, that may be activated using another chemical, and thereby, in a controlled process, release energy and a rapidly expanding gas. Hydrocarbon-based fuels, such as gasoline, gasoil or diesel, may also be used as reagents and be supplied through a flowline.

The solid fuel gun comprises solid fuel, such as charcoal, graphite or cordite, and potassium nitrate or sodium nitrate. The solid fuel may also be mixed with sulphur. The solid fuel gun is ignited by arc ignition.

In order to ease the accumulation of oil-containing fluid even further while sending the mechanical waves into the formation, the injection fluid is hot fluid having a temperature at a point of injection downhole which is higher than that of the formation. The temperature of the hot fluid is at least 10° C. higher than the temperature of the formation, preferably at least 25° C. higher than the temperature of the formation, and more preferably at least 50° C. higher than the temperature of the formation. In some wells, the temperature of the hot fluid is at least 150° C., preferably at least 175° C., and more preferably at least 200° C. in order that the hot fluid temperature is higher than the formation temperature.

The injection fluid is gas, such as methane gas or carbon dioxide, or water, such as sea water.

In FIG. 6, the stimulation system comprises 10 production wells 2 and 18 injection wells 1, wherein some of the injection wells are periphery injection wells encircling at least one production well and at least one non-periphery injection well. The periphery injection wells are marked by a dotted line 27 in FIG. 6. The activation devices in the periphery injection wells are activated before the other injection well and may also be activated more frequently in order to encircle the oil-containing fluid and force the oil-containing fluids towards the production wells 2.

When having injection fluid injected below the oil-containing fluid, the non-periphery injection wells are activated more frequently than the periphery injection wells due to the fact that the fluid surrounding the production wells 2 are drained from the formation, and therefore room is provided for the injection fluid to find its way to the production zone as illustrated in FIG. 4 a.

Before determining the activation pattern, which is the order in which activation devices in a given injection well and/or production well are to be activated, and determining the frequency of the activation, the water cut and also the water hold are determined using a water cut meter and a flow meter in at least the production well. The activation of activation devices may be performed even though the production of production wells 2 is satisfactory in order to prevent the production from decreasing or the water cut from increasing. The activation frequency of the activation devices may be increased if the water cut is above a preselected range or decreased if the water cut is below a preselected range.

By activating activation devices continuously with the predetermined frequency, the production is optimised, meaning that the water cut is kept at an optimal level. By having such continuous activation, it is possible to bring up more oil-containing fluid from the oil field than by means of conventional methods and to increase the percentage of reservoir oil which the oil-producing company is able bring up from a reservoir. Presently, when oil is recovered, only a maximum of 40% is brought up. The rest is left in the reservoir, and by bringing up the 40%, the reservoir may be disturbed to a degree where it is not possible to bring up the remaining 60%. Therefore, there has been a long-felt need to increase this percentage.

In FIG. 7 a, the production well 2 has a plurality of openings in a first production zone 10 a, and in a second production zone 10 b the production well comprises other openings. Inlet valves 7, 7 a are arranged in the openings in the first production zone 10 a, and in the openings in the second production zone inlet valves 7 b having different inlet flow settings from the valve of the first production zone are arranged. Hereby, a pressure gradient is created in a region 8 of the formation between the two production zones illustrated by a dotted line area, and by arranging the activation devices 3 transmitting mechanical waves into the region of the formation having the pressure gradient, oil-containing fluid is released in that region as micro bores are created, enabling the oil-containing fluid to flow and accumulate into greater pools. The production zones are separated by means of annular barriers 9.

In FIG. 7 b, the activation device 3 is arranged in an injection well 1 between two injection sections 5 a, 5 b having different outlet flow settings at the openings 5 in the casing 25. The two outlet sections 5 a, 5 b, where one outlet section 5 a has a different flow setting than the other outlet section 5 b, create the pressure difference in the region 8 between the two injection sections 5 a, 5 b. The activation device 3 transmits mechanical waves into the region 8 having the high pressure gradient, thereby creating micro bores in the formation, particularly in sandstone or limestone formation, and thus releases oil trapped therein.

Water injection typically leads to an increase in the amount of oil which may be extracted from a reservoir; however, at some point, water injection will not be able to force any more oil out of the reservoir, leading to an increase in the water cut. The increase in water cut may originate from the water injection or from water presence close to the reservoir. At this point or even before, mechanical waves, through such part of the formation, may energise the formation such that oil droplets or particles in the formation may gain enough energy to escape surfaces binding the oil droplets or particles in the formation, thereby allowing them to be dissolved in the free-flowing fluids in the formation, e.g. injection fluid. This may further increase the oil production in the reservoir, leading to a decrease in the water cut of the oil-containing fluid in the production wells. At very high energies of the mechanical waves or close to certain Eigen frequencies of parts of the formation, the formation may be forced to crack, fracture or splinter, allowing oil droplets or particles to escape closed oil pools, closed micro bores in the formation or other closed volumes in the formation, thereby increasing the level of oil in the oil-containing fluid.

As shown in FIG. 5, the activation device may be powered and controlled via a wireline 18. In this way, the activation of the activation devices can be controlled from the top of the well, and the activation pattern can easily be changed from surface if the water cut has changed. But also activation devices pre-integrated in well tubular structures of injection wells during completion may be appropriate and activated from the surface, e.g. by propagation of pressure waves through the injection fluid present in the injection well.

In the event that the activation device is not submergible all the way into the casing, a driving unit such as a downhole tractor can be used to push the tools all the way into position in the well. A downhole tractor is any kind of driving tool capable of pushing or pulling tools in a well downhole, such as a Well Tractor®. The downhole tractor comprises wheels arranged on retractable arms.

Although the invention has been described in the above in connection with preferred embodiments of the invention, it will be evident for a person skilled in the art that several modifications are conceivable without departing from the invention as defined by the following claims. 

1-26. (canceled)
 27. A stimulation system for stimulation of oil production in an oil field, comprising: a plurality of wells, and a plurality of activation devices arranged in the wells, wherein the activation devices are activated with a frequency of once within a period of 1-365 days and with an energy discharge of at least 0.1 kilograms TNT equivalence per activation, characterised in that the activation device is a fluid-activated gun, said fluid being pressurised injection fluid, and the gun converts energy from the pressurised fluid into mechanical waves, where said gun is activated continuously in a time interval during the period, providing vibrations having an energy of at least 0.1 kilograms TNT equivalence in total during the period.
 28. A stimulation system for sitimulation according to claim 27, wherein the activation devices are reusable, i.e. the activation devices can be used several times, eliminating the need to pull the activation devices out of the well for recharging.
 29. A stimulation system according to claim 27, wherein the activation devices are activated with the frequency of once within a period of 1-185 days, preferably once within a period of 1-90 days, more preferably once within a period of 1-30 days, and even more preferably once within a period of 5-20 days.
 30. A stimulation system for stimulation according to claim 27, wherein the activation devices are activated at intervals of at least 5 days, preferably at intervals of at least 10 days, more preferably at intervals of at least 15 days.
 31. A stimulation system for stimulation according to claim 27, wherein the wells are both a plurality of production wells and a plurality of injection wells, the plurality of activation devices (3) being arranged in the injection wells and/or production wells.
 32. A stimulation system for stimulation according to claim 27, wherein the activation devices are activated in a predetermined pattern determining in which well the activation device is activated.
 33. A stimulation system for stimulation according to claim 27, wherein the first activation device is activated on a first day of the period, and the second activation device is activated on another day of the period.
 34. A stimulation system for stimulation of oil production in an oil field, comprising: a plurality of wells, and a plurality of activation devices arranged in the wells, wherein the activation devices are activated with a frequency of once within a period of 1-365 days and with an energy discharge of at least 0.1 kilograms TNT equivalence per activation, characterised in that the activation device is a fluid-activated gun, said fluid being pressurised injection fluid, and the injection fluid has a temperature at a point of injection downhole which is higher than that of the formation.
 35. A stimulation system according to claim 27, further comprising: a plurality of openings in at least one of the wells, wherein at least two neighbouring openings have different inlet flow settings, wherein the activation devices are arranged between said two neighbouring openings having different inlet flow settings for transmission of mechanical waves into a region of the formation having a high pressure gradient, thereby releasing oil in said region.
 36. A stimulation method comprising the steps of: arranging a plurality of activation devices in a plurality of wells, and characterised by the step of: activating the activation devices which are fluid-activated guns, said fluid being pressurised injection fluid, with a preselected range of frequencies or a single frequency.
 37. A stimulation method according to claim 36, wherein a first activation device arranged in a first well is activated in the first well with a frequency of once within a period of 1-365 days, and a second activation device arranged in a second well is activated in the second well with a frequency of once within a period of 1-365 days.
 38. A stimulation method according to claim 36, wherein the activation devices are activated with an energy discharge of at least 0.1 kilograms TNT equivalence per activation.
 39. A stimulation method according to claim 36, wherein the activation devices are activated with a frequency of once within a period of 1-365 days.
 40. A stimulation method according to claim 36, wherein the activation devices are activated with a frequency of once within a period of 1-185 days, preferably once within a period of 5-90 days, more preferably once within a period of 7-30 days, and even more preferably once within a period of 7-20 days.
 41. A stimulation method according to claim 36, comprising the step of: activating the activation devices in a predetermined pattern determining in which well an activation device is activated.
 42. A stimulation method according to claim 36, comprising the steps of: activating a first activation device of the plurality of activation devices encircling at least one production well, activating a second activation device positioned substantially furthest away from the first activation device and on the opposite side of the at least one production well, activating a third activation device positioned substantially furthest away from the second activation device and on the opposite side of the at least one production well, and activating a fourth activation device positioned substantially furthest away from the third activation device and on the opposite side of the at least one production well and so forth until all activation devices of the plurality of activation devices are activated, and then activating the plurality of activation devices once more a predetermined number of times.
 43. A stimulation method according to claim 36, wherein the first activation device is activated on a first day of the period, and the second activation device is activated on another day of the period.
 44. A stimulation method for stimulation according to claim 36, wherein the activation device is a fluid-activated gun, said fluid being pressurised injection fluid, and the gun converts energy from the pressurised fluid into mechanical waves, where said gun is activated several times during the period, providing vibrations having an energy of at least 0.1 kilograms TNT equivalence in total during the period.
 45. A stimulation method according to claim 36, further comprising the step of: arranging a plurality of activation devices in a plurality of periphery injection wells, said periphery injection wells encircling at least one production well and at least one non-periphery injection well.
 46. A stimulation method according to claim 36, further comprising the steps of: setting an inlet flow of a plurality of inlet valves in openings in a first production zone so that inlet valves in openings in a second and neighbouring production zone have different inflows, thereby creating a pressure gradient in a region of the formation between said plurality of inlet valves, and arranging and activating the activation device in the well opposite the region of the formation between said plurality of inlet valves having different inflows, thereby releasing oil in said part of the formation. 